The invention relates to devices for rotating an object about an axis, more especially to rotation devices that operate by the principle of inertial slip-stick motion.
Rotational positioning devices are in widespread use in diverse industrial and scientific applications. Applications that use rotational positioners include: various forms of microscopy, including scanning probe microscopy, optical microscopy and electron microscopy; sample handling during semiconductor processing; a variety of vacuum applications, including ultra-high-vacuum (UHV) applications; rotation of objects in a magnetic field generated by a conventional or superconducting magnet; and a variety of applications related to rotating samples or other components inside cryostats, including magnet cryostats.
Rotational positioning devices may be based on a number of different classes of design. For example, the devices may be based on stepper motors. Another class of designs, which is used for camera auto-focus applications, is based on ultrasonic motors. A further class of designs, to which the present invention belongs, is based on inertial motors. A known rotational positioner based on an inertial motor is now described.
FIGS. 1A and 1B of the accompanying drawings shows an inertial rotational positioner as described in reference [1]. A polished steel sphere S lies on three piezoelectric actuators A, B and C which are fixed to a common base plate X. The sphere constitutes the movable, i.e. rotatable, part of the positioner. For achieving a small and well-defined contact area between the actuators and the steel sphere, a glass or ruby ball R is fixed at the end of each actuator. A magnet M is fixed to the base plate X and arranged to pull down the steel sphere onto the actuators by a magnetic force of five to ten times the weight of the sphere S. This clamping force allows the motor to generate larger torques, as will be understood from general principles of slip-stick motion. The device is capable of providing three rotational axes 1, 2 and 3.
To achieve a small amount of rotation about axis 1, 2 or 3, each actuator has to contribute an appropriate tangential component. The directions of these components are drawn as arrows in the figures, and labelled a1, b1 and c1 for the respective actuators A, B and C. To rotate the sphere S, the piezoelectric actuators are actuated in bending mode with the bending direction being specific to the desired rotation direction. The amount of displacement, that is the size of the bending, must also be chosen correctly. Each positioning step is composed of two events. First a slow bending of all actuators simultaneously in the directions of arrows a1, b1, c1 and then an abrupt jump back of the actuators to their original straight alignment. During the slow bending, the sphere follows the actuators by friction, whereas during the abrupt jump back the sphere cannot follow the actuators because of its inertia. The actuators thus return to their original straight alignment under slippage between the actuators and the sphere. These two events are the “stick” and “slip” phases that typify any inertial or “slip-stick” motor. A series of such stick and slip steps can thus be used to rotate the sphere about axis 1, 2 or 3 through a desired angle.
This design has been used to control a mirror mounted on the sphere in a galvanometric system to detect cantilever deflection in an atomic force microscope. The design is UHV compatible.
The prior art design does however have some limitations.
Like all inertial positioners, step size is not very reproducible and is load dependent. In a prior art linear inertial positioner, such as described in reference [2], variation in step size does not compromise the geometry, i.e. direction, of the motion, only the magnitude of the motion. Although not desirable, this variation in the magnitude of the motion in the design of reference [2] can be compensated for by some kind of feedback. However, in the design of reference [1], the step size fluctuations that are inevitable in an inertial positioner will result in wandering of the axis of rotation. In other words, the step size fluctuations will not only result in the angular displacement deviating from the desired magnitude, but also in the geometry of the rotational motion being inaccurate. This is more difficult to compensate for than a variation in the magnitude of the motion. The problem may be viewed as being a result of the fact that, in the design of reference [1], there is no one-to-one correspondence between the three actuators and the three rotational axes. This limitation is inherent to the design, since it follows from the sphere's geometry.
The drive electronics needed for implementing the design of reference [1] are also relatively complicated, since the three actuators need to be supplied with separate drive signals that are carefully co-ordinated. This limitation also follows from the lack of one-to-one correspondence between the actuators and the rotational axes.
Other inertial motor designs are described in reference [3-8].